Pulsed fiber-MOPA with widely-variable pulse-duration

ABSTRACT

Master oscillator power amplifier (MOPA) apparatus includes two seed-pulse sources coupled to a single fiber amplifier including one or more stages of amplification. One of the seed-pulse sources is a single-mode source generating pulses having a duration selectively variable between about 0.1 ns and 10 ns. The other seed-pulse source is a multi-mode source generating pulses having a duration selectively variable between about 1 ns and 10 μs. Selectively operating one or the other of the seed-pulse sources provides that the apparatus can deliver pulses selectively variable in a range between about 0.1 ns and 10 μs.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to master oscillator power amplifier(MOPA) laser systems including fiber-amplifiers. The invention relatesin particular to pulsed MOPAs in which pulse duration is selectivelyvariable.

DISCUSSION OF BACKGROUND ART

Extra cavity frequency-converted pulsed solid state lasers are usedextensively for material processing applications such as machining,drilling, and marking. Most commercially available, pulsed, solid-statelasers are Q-switched pulsed lasers. Q-switched pulsed lasers include alaser-resonator having a solid-state gain-element and selectivelyvariable-loss device located therein. The laser resonator is terminatedat one end thereof by a mirror that is maximally reflecting at afundamental wavelength of the gain-element, and terminated at anopposite end thereof by a mirror that is partially reflecting andpartially transmitting at the fundamental wavelength. Such a laser isusually operated by continuously optically pumping the gain elementwhile periodically varying (switching) the loss caused by the variableloss device (Q-switch) between a value that will prevent lasing in theresonator and a value that will allow lasing in the resonator. Whilelasing is allowed in the resonator, laser radiation is delivered fromthe partially transmitting mirror as a laser pulse.

The pulse repetition frequency (PRF) of a Q-switched solid-state laseris determined by the frequency at which the Q-switch is switched. Thepulse duration is determined for any particular gain-medium by factorsincluding the length of the resonator, the transmission of thepartially-transmitting mirror, losses in the Q-switch in a“lasing-allowed” condition, the optical pump power, and the PRF. A pulserepetition rate and pulse duration that are optimum for an operation onany one material will usually not be optimum for another operation oranother material. Accordingly, an “ideal” pulsed laser would haveindependently variable PRF and pulse-duration to allow an optimumcombination to be selected for most operations on most materials.

One type of laser system in which the PRF can be varied without avariation in pulse duration is a fiber-based MOPA in which seed pulsesare generated by a master oscillator in the form of a modulated,edge-emitting semiconductor laser (diode-laser) and amplification isprovided by a fiber-amplifier. A fiber-amplifier has relatively highgain, for example between about 13 decibels (dB) and 30 dB. This,combined with a low saturation power, allows a variety of low-powerdiode-laser seed sources to be used. Such a fiber MOPA can be operatedat PRFs from less than 100 kilohertz (kHz) to 5 Megahertz (MHz) orgreater, with pulse durations selected between about 0.1 nanosecond (ns)and about 1 microsecond (μs).

A significant problem in fiber-amplifiers is created by nonlineareffects in fibers which limit peak power and affect the spectralcharacteristics of the optical pulses. For harmonic generation atnanosecond pulses, spectrally-narrow light with a bandwidth betweenabout 0.5 nanometers (nm) and 1.0 nm is required. Stimulated Ramanscattering (SRS), stimulated Brillouin scattering (SBS), and spectralbroadening of nanosecond pulses due to four-wave mixing (FWM) in fiberssignificantly narrow the available space of optical parametersacceptable for frequency conversion.

Stimulated Raman scattering (SRS) limits the peak power in typicalall-fiber systems with core diameters below 30 μm (so called LMA fibers)although larger diameters of up to 100 μm are also possible usingspecially designed fibers (so called photonic-crystal fibers andleaky-mode fibers). SRS is the only power-limiting factor for broadband,for example greater than 1.0 nm bandwidth, IR nanosecond pulses.Attempts to narrow the spectral bandwidth using a single-frequency seedsource encountered a build-up of stimulated Brillouin scattering (SBS)resulting in optical damage of fiber components.

It is known that for long optical pulses, for example pulses having aduration greater than 20 ns with a bandwidth much broader than for SBS,for example between about 0.20 picometers (pm) and 0.25 pm (betweenabout 40 and 50 MHz), the threshold power for SBS grows proportionallyto the signal spectral width. This is why a common approach for SBSsuppression in long pulses is to broaden the pulse bandwidth to be muchlarger than the SBS bandwidth of about 0.2 pm while keeping the pulsebandwidth below about 1 nm that is appropriate for frequency conversion.However, spectral broadening of nanosecond pulses due to four-wavemixing in fibers transfer the energy from the narrow spectral peak tothe spectral wings at pulse peak powers above 100 W (FIG. 1). Thiseffect reduces frequency-conversion efficiency in all-fiber systemscompared to solid-state lasers.

Another way to reduce SBS is to use pulses shorter than the SBS build-uptime in fibers, which is typically close to 20 ns. For pulses having aduration less than 20 ns, SBS occurs in a transient regime with asmaller gain-factor.

There are two common approaches to generate pulses with variable lengthand pulse repetition rate. The first approach uses a directlymodulated=diode-laser as a seed source. Such an approach is, in general,less expensive, and provides high peak power, for example greater than 1Watt (W) from the seed laser. A major disadvantage of this approach isthat to provide short pulses having a duration of less than 10 ns ashort cavity length, for example less than about 10 millimeters (mm), isrequired. This, in turn, results in a single-frequency or “few-frequencymode” operation that favors SBS and limits the peak power infiber-amplifiers. For this reason, this approach is limited to shortpulses (duration less than about 10 ns), where SBS exhibits a reducedgain. Using broadband reflectors in the cavity and generating more modeshelps to reduce SBS but immediately results in stronger broadening ofthe spectrum due to FWM, making frequency-conversion inefficient.

The second approach uses a continuous wave (CW) optical source or pulsedoptical source with long pulses, modulated by an external modulator. Insuch a system, a seed source could be a diode-laser, a solid-statelaser, a fiber laser, or a source generating amplified spontaneousemission (ASE source), such as a superluminescent LED, while a typicalmodulator is an electro-optical crystal in the waveguide Mach-Zehnderconfiguration or a semiconductor optical amplifier.

On one hand, such an approach provides less peak power (typically lessthan 100 mW) after modulation compared with that available from adirectly modulated diode. On the other hand this approach allowsgeneration of pulses of any length and repetition rate with a spectrumdetermined by an appropriately designed seed laser. For example, a diodeseed laser may have a low noise operation when a fiber Bragg grating(FBG) written in a fiber placed in 1-2 m from a diode-laser chipprovides an output coupler for the diode-laser cavity. With a typicalFBG bandwidth of between about 0.01 nm and 1.0 nm, such a diode-laserwill have a much broader spectrum than the SBS bandwidth, which helps tosuppress SBS. However, due to mode-beating such a source exhibitspulse-to-pulse fluctuations, especially when operating with short pulsesof less than 10 ns duration. The second approach is therefore limited topulse durations of greater than about 10 ns.

For many applications it would be desirable to have a fiber MOPA with afrequency conversion stages operating at any pulse duration betweenabout 0.1 ns and 1 microsecond.

SUMMARY OF THE INVENTION

In one aspect of the present invention optical apparatus comprises anoptical amplifier including one or more fiber-amplification stages. Asingle-mode source of optical pulses is provided and arranged to deliveroptical pulses having a pulse-duration selectable in a first range ofpulse durations including pulse durations between a first duration and asecond duration. A multi-mode source of optical pulses is provided andarranged to deliver optical pulses having a pulse-duration selectable ina second range of pulse durations including pulse durations betweenabout a third duration and a fourth duration, where the fourth durationis greater than the second duration. Pulses from the few mode andmulti-mode pulse sources can be selectively coupled to the opticalamplifier, such that the apparatus can deliver optical pulses having aduration selectable in a third range of durations between the firstduration and the fourth duration.

In a preferred embodiment of the invention the first, second third andforth durations are respectively about 0.1 ns, 10 ns, 1 ns, and 1000 ns.Accordingly, in this embodiment of the inventive apparatus can deliverpulses having a duration selectable in a range between about 0.1 ns and1000 ns (1 μs).

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, schematically illustrate a preferredembodiment of the present invention, and together with the generaldescription given above and the detailed description of the preferredembodiment given below, serve to explain principles of the presentinvention.

FIG. 1 schematically illustrates one preferred embodiment of MOPAapparatus in accordance with the present invention having a single modeseed pulse source including a contrast-enhancing modulator and amulti-mode seed-pulse source with both sources coupled to a fiberamplifier including two stages of fiber amplification, a pulse durationand source selector cooperative with the seed pulse sources to selectthe source and duration of pulses delivered to the fiber amplifier, anda frequency converter for changing the wavelength of amplified pulses.

FIG. 2 schematically illustrates another preferred embodiment of MOPAapparatus in accordance with the present invention, similar to theapparatus of FIG. 1, but wherein the contrast enhancing monitor of thesingle-mode seed pulse source is replaced by a contrast enhancingmodulator in the fiber amplifier.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like components are designated bylike reference numerals, FIG. 1 schematically illustrates one preferredembodiment 10 of a fiber-MOPA laser apparatus in accordance with thepresent invention. Apparatus 10 includes two optical sources 12 and 14.The optical sources are connected via a coupler 20 to one or morefiber-amplifier stages. Two stages 22 and 24 are depicted in FIG. 1 byway of example. The amplifier output is directed to one or morefrequency conversion stages 26 for converting the fundamental wavelengthof the amplified pulses to a different wavelength such as the second,third, or fourth, harmonic wavelength. Those skilled in there to whichthe present invention relates will recognize that in any multistageamplifier arrangement it is advisable to provide an optical isolatorbetween stages to prevent feedback for one stage into a prior stage orfrom an amplifier stage back to a source. Isolators are not shown inFIG. 1 for simplicity of illustration. Non-linear crystals can be usedfor the frequency conversion stage.

Optical source 12 is a single-mode pulsed source which provides opticalpulses having a duration selectively variable in a relativelyshort-duration range, for example between about 0.1 ns to 10 ns. Here“single-mode” source is intended include a source that operates in oneor a very few (four or less) modes. However, pure single mode operationis preferred. Optical source 12 can be a single-mode (single-frequency)directly modulated diode-laser, in particular an extended cavitydiode-laser with distributed feedback (DFB) provided by distributedBragg grating (DBG) written in an optical fiber (detail not shown). Insuch a directly modulated diode-laser, there is a small frequency sweep(chirp) from the beginning of the pulse to the end of the pulse. Thisfrequency chirp helps to increase SBS threshold. Single-mode operationof source 12 also helps to reduce FWM in fiber-amplifiers and providesgood pulse stability. A combination of a CW single-frequency laser withan external modulator can be used instead of a directly modulateddiode-laser for source 12.

Optical source 14 is a multiple-longitudinal-mode (multiple-frequency)pulsed laser providing radiation having a spectral bandwidth muchbroader, for example at least about ten-times broader, than the SBSbandwidth of amplifier fibers but less than about 1 nm. Preferably, thespectral bandwidth (FWHM) is between about 50 pm and 1 nm. Laser 14provides optical pulses having a duration selectively variable in arange of durations longer than the range of laser 12, for examplebetween about 1 ns to 1000 ns (1 μs). For such a broadband source, theSBS threshold grows proportional to the spectral bandwidth of thepulses. Here it should be noted that there can be some overlap betweenthe pulse duration ranges of sources 12 and 14.

Optionally, the output of the source can be modulated by a modulator 16to increase contrast between pulses and background light. This techniqueis described in detail in U.S. patent application Ser. No. 12/132,449,filed Jun. 3, 2008, assigned to the assignee of the present inventionand the complete disclosure of which is hereby incorporated byreference.

Further it may be necessary to locate either an amplifier or anattenuator between either of sources 12 and 14 and the coupler,depending on the sources and other components, to approximately matchaverage powers from both sources at the coupler. By way of example, inapparatus 10, a fiber-amplifier 18 is included between source 14 and thecoupler to compensate extra loss caused by optical modulator 16.

An extended cavity diode-laser with an output mirror based on a fiberBragg grating having a bandwidth of more than 0.01 mn and placed atleast 1 meter from the diode chip is one example of a laser suitable formultimode source 14. Another example of a laser suitable for multimodesource 14 is an amplified spontaneous emission (ASE) fiber-laserincluding a spectral filter (for example, a fiber Bragg grating), whichnarrows the bandwidth of ASE source while keeping the coherence lengthof the source short. Such an ASE fiber-laser having short coherencelength would help to reduce FWM and increase SBS threshold infiber-amplifiers 22 and 24. It is preferable that pulses from bothsources have the same nominal wavelength (center wavelength)

Coupler 20 combines light from each of sources in one fiber 21 anddelivers it to amplifier stages 22 and 24. Preferably coupler 20 is afused-fiber coupler or a fiber-pigtailed micro-optics based coupler.Only one of sources delivers pulses to fiber amplifiers 22 and 24 at anytime depending on the desired pulse duration. The source and pulseduration is selected by user input to a selector 28, which can be acomponent of a more universal control-electronics arrangement foroperating parameters of apparatus 10. Alternative possible couplingarrangements include such a mechanical switch between fiber output portsof each source, an electro-optical modulators such as a Mach-Zehnder(MZ) interferometer formed in a lithium niobate (LiNbO₃) crystal, and anacousto-optic (A-O) modulator with two input ports an one output port.

FIG. 2 schematically illustrates another preferred embodiment 30 of afiber-MOPA laser apparatus in accordance with the present invention.Apparatus 30 is similar to apparatus 10 of FIG. 1, with an exceptionthat modulator 16 in the single mode source arm of apparatus 10 isomitted and an optional modulator 32 is located between coupler 20 andfiber-amplifier stage 22. Modulator 32 provides contrast enhancement forpulses from either of sources 12 and 14, or could provide additionalenhancement if either source included a contrast enhancing monitor. Itshould be noted, however, that the contrast-enhancement action of themonitor reduces the duration of pulses the contrast of which is beingenhanced. This should be taken into account when selecting apulse-duration from either source. Modulator 32 can also be used fortemporally shaping pulses from either source.

The present invention is described above with reference to preferredembodiments thereof. The invention however is not limited to theembodiments described and depicted herein. Rather the invention islimited only by the claims appended hereto.

What is claimed is:
 1. A laser system for generating a pulsed outputcomprising: a single mode source for generating optical seed pulseshaving a first bandwidth; a multimode source for generating optical seedpulses having a second bandwidth larger than the first bandwidth; afiber amplifier for selectively amplifying pulses from one of either thesingle or multimode sources; a frequency conversion stage for convertingthe frequency of the amplified pulses; and a controller for selectivelyactivating one of the two sources to generate pulses having a desiredrepetition rate, wherein the single mode source can be selected forpulses having a duration in a first range and the multimode mode sourcecan be selected for pulses having a duration in a second range, with thefirst range encompassing pulse durations shorter than the second range.2. The apparatus of claim 1, wherein the single-mode optical-pulsesource includes a single-mode diode-laser.
 3. The apparatus of claim 2,wherein the single-mode diode-laser is a directly modulated diode-laser.4. The apparatus of claim 1, wherein the multi-mode optical-pulse sourceincludes an extended cavity diode-laser.
 5. The apparatus of claim 4,wherein the extended cavity diode-laser has a cavity length about equalto or greater than 1 meter.
 6. The apparatus of claim 4, wherein theextended cavity diode-laser is a directly modulated diode-laser.
 7. Theapparatus of claim 1, wherein pulses delivered by the multimode sourcehave a spectral bandwidth between about 50 picometers and 1 nanometer.8. A laser system as recited in claim 1, wherein the beandwith of thepulses generated by the multimode source is at least 50 picometers.
 9. Alaser system as recited in claim 8, wherein the duration of the pulsesgenerated by the single mode source ranges from about 0.1 nanoseconds to10 nanoseconds and the duration of the pulses generated by the multimodesource ranges from about 1 nanosecond to about 1000 nanoseconds.
 10. Alaser system for generating a pulsed output comprising: a single modesource for generating optical pulses; a multi-mode source for generatingoptical pulses having a bandwidth of at least 50 picometers; a fiberamplifier for selectively amplifying pulses from one of either the firstor second sources; a frequency conversion stage for converting thefrequency of the amplified pulses; and a controller for selectivelyactivating one of the two sources to generate pulses having a desiredrepetition rate and duration wherein single mode source can be selectedfor pulses having a duration in a first range and the multimode modesource can be selected for pulses having a duration in a second range,with the first range encompassing pulse durations shorter than thesecond range.
 11. A laser system as recited in claim 9, wherein theduration of the pulses generated by the single mode source ranges fromabout 0.1 nanoseconds to 10 nanoseconds and the duration of the pulsesgenerated by the multimode source ranges from about 1 nanosecond toabout 1000 nanoseconds.
 12. A laser system as recited in claim 9,wherein the controller causes the selected source to generate pulseshaving a desired repetition rate, with the source and the duration ofthe pulses being selected to minimize scattering in the fiber amplifierand maximize conversion efficiency in the frequency conversion stage.13. A method of operating a laser system, said laser system including afirst laser generating single mode optical seed pulses having a firstbandwidth and a second laser generating multimode optical seed pulseshaving a second bandwidth larger than the first bandwidth, said lasersystem including a fiber amplifier for selectively amplifying pulsesfrom one of either the first or second lasers, said laser systemincluding a frequency conversion stage for converting the frequency ofthe amplified pulses, said method comprising; selectively activating thefirst laser to generate single mode pulses having a duration in a firstrange and, alternatively, selectively activating the second laser togenerate multimode pulses having a duration in a second range, with thefirst range encompassing pulse durations shorter than the second range.14. A method as recited in claim 13, wherein the bandwidth of the pulsesgenerated by the second multimode laser is at least 50 picometers.
 15. Amethod as recited in claim 13, wherein the duration of the pulsesgenerated by the first single mode laser ranges from about 0.1nanoseconds to 10 nanoseconds and the duration of the pulses generatedby the second multimode laser ranges from about 1 nanosecond to about1000 nanoseconds.